To transfer the optical signal from one fiber cable to another one, the output and input cables' ends must be attached to output and input connectors of appropriate types; for example: males and/or females. Depending on the output signal type Single Mode (SM) or Multi Mode (MM), the appropriate input connector must be used. Currently, the main problem is that, for different types of output connectors, people use the appropriate different types of input connectors because the geometry of output connectors are different.
In a typical MPO type connector, up to twelve fibers in a ribbon are stripped to 125 μm cladding and inserted into 250 μm spaced parallel alignment holes. The ferrule also typically includes two 0.7 mm diameter holes, running parallel to the fibers on the outer side of the ferrule. These two holes hold precision metal guide pins which align the fibers with tight tolerances. MPO type connectors may be male and female; male connectors have two guide pins and female connectors do not. Because typical MPO connectors are trying to align so many fibers at once, their coupling loss are typically larger than single fiber connectors.
FIGS. 1A and 1B represent the current technical level, in which we see the male type connector (1A) and the female type connector (1B). The surfaces of the appropriate ferrules (94 and 94) of the connectors (88 and 90), after they are connected, must be parallel and in strong coincidence with each other. If a MM type signal is used, the surfaces of each ferrule (92, 94) are parallel to each other, are flat and perpendicular to the direction of the signal propagation. If a SM type signal is used, the surfaces of each ferrule (92, 94) are parallel to each other, are flat, but are slightly angled/wedged 8 degrees toward the direction of the signal propagation. However, the main problem in practice is, that for each type of signal (for example: MM, SM), a different type of ferrule (94) has to be used. Current technology does not include a ferrule having a geometry that could be suitable for both types (MM and SM) of signal going from output fiber.
Looking at the current technology in more detail, there are two basic ferrule types for MPO connectors; a flat polished ferrule that is perpendicular to the direction of the light propagation and a flat and angled polished ferrule that is angled to the direction of the light propagation. The flat and perpendicular ferrule is used in case of MM signal fibers, which may be referred to as a MM type ferrule. The flat and angled/wedged angle ferrule is used in case of SM signal fibers, which may be referred to as a SM type ferrule. The wedged surface of the ferrule is disposed at an 8° angle, which is designed to minimize the signal reflections. Respectively, the connectors in which MM type ferrules are used are called as MM type connectors, and the connectors in which SM type ferrules are used are called SM type connectors. Also, by functionality, the connectors can be divided to “output” and “input”. In current technology, the angled (SM type) and flat (MM) polished connectors are not designed to be connected to each other. Connecting a flat polished ferrule to an angle polished ferrule results in high losses of the optical signal intensity and risk of mechanical damage of the surfaces of the ferrules.
There are used two main parameters related to losses in fiber optical connectors: insertion losses and reflection losses. Insertion losses refer to the loss of signal, or reduction of its intensity, at the entrance of the connection and is normally specified in decibels (dB). Insertion losses should be low; typically a few hundredths to a few tenths of a dB. Reflection losses refer to the amount of tight reflected back to the originating source or to the environment around. Reflection losses should be as low as possible indicating a small amount of light being reflected back. Reflections of light can cause damage in the transmission system and in testing optical systems. Typical reflection losses specifications range from minus 20 to minus 65 the values depend on fiber and connector type.
The losses inherent in the connection of flat polished to angle polished ferrules are largely due to the alignment of the optical fiber, whose end is embedded in the ferrule of the connector. FIG. 2 shows a perspective diagram depicting a light acceptance cone (10) being released from an optical fiber (20) with core (22) and cladding (24). FIG. 3 shows an angle polished ferrule (2) being connected to a flat polished ferrule (4). Angle polished ferrule (2) includes an optical fiber with core (12) and cladding (14), ending in fiber end (6). Flat polished ferrule (4) includes an optical fiber with core (16) and cladding (18), ending in fiber end (8). Light is being released from the core (12) in fiber end (6) embedded in angle polished ferrule (2) into the fiber end (8) embedded in flat polished ferrule (4). As shown in FIG. 3, the wider end of light acceptance cone (10), formed out of fiber end (6), is wider than fiber end (8), causing substantial insertion losses at the entrance of the connection.
To avoid such situations, it is common for flat polished connectors to mate with connectors with flat ferrules, as shown in FIG. 4A and angle polished connectors to mate with connectors with angled ferrules, as shown in FIG. 4B. In scenarios such as those shown in FIGS. 4A and 4B, the ferrules are in physical contact, so the cores of the optical fibers within each of the connecting ferrules may be the same size without substantial insertion loss because the light being released from one fiber end will not form a substantial light acceptance cone, but rather will directly enter the connecting fiber end. In some cases, physical contact between the ferrules is undesirable as it may abrade the ferrule ends, which may cause distorted measurements.
Although FIGS. 4A and 4B show actual physical contact between the connectors, there may also be a gap between the connectors, as shown in FIG. 4C. Having a gap avoids issues caused by physical con tact between the ferrules, as discussed above. In scenarios that include a gap, as shown in FIG. 4C, the core of the optical fiber that receives light at the connection must be at least as wide as the light acceptance cone produced by the releasing fiber end at the light acceptance cone's widest point, where it meets the fiber end of the optical fiber accepting the light, to avoid substantial insertion loss. Thus, the distance between the fibers must be relatively small. FIG. 4D shows the result when the distance d between two flat polished connectors is too large. In FIG. 4D, the light 0 from output fiber 1 passes through fiber end A toward the fiber end B of input fiber II. A portion 1 of the light 0 passes through fiber end B and into input fiber II, and a portion is reflected off of fiber end B. This light then reflects back off of fiber end A toward fiber end B, where still another portion 2 passes into the input fiber II. Such reflections cause signal interference and result in distorted measurements. To avoid interference between portions of light 2 and 1, the angle between them must be large enough. To fulfill this condition, the surface of one of the fiber is wedged. However it results in potential abrasion problems.
There are some applications where it is necessary, using one universal input connector, to make a connection with output connector of any type (SM or MM). A primary example of such an application is a field of testing equipment. Such mentioned one universal input connector can reduce the cost of all testing equipment and simplify the testing process itself. Therefore there is a need for a hybrid ferrule that is able to be connected to both, angle polished and flat polished, MPO connectors while, at the same time, minimizing insertion and reflection losses.